Integrated circuit photofabrication masks and methods for making same

ABSTRACT

An attenuated phase shifting mask employs regions of increased light transmissivity adjacent the defined circuit pattern features. Such regions can be provided by partially oxidizing a secondary region of the halftone masking layer. The result is improved image resolution and depth of focus, and a minimization of image shortening effects. In a second primary embodiment, similar improvements, as well as well as sharper corner definition, are obtained by providing on a mask (conventional or phase shifting) a generally rounded, light diffracting topography at edges of the defined circuit pattern features. This can be accomplished, for an elongated hole feature, by depositing a layer of light transmissive material on a conventional mask structure to form a generally convex light transmitting surface overlying an edge of the masking layer. In the case of a line feature, the substrate can be etched to form a recessed region including a generally concave light transmissive surface extending beneath an edge portion of the masking layer.

BACKGROUND OF THE INVENTION

The present invention relates to masks used in the production ofsemiconductor integrated circuits (IC's). More specifically, the presentinvention relates to mask structures and production methods forimproving image resolution and depth of focus, and reducing imageshortening effects, in photolithographic IC production (i.e., opticalphotofabrication).

Today, most semiconductor integrated circuits are formed utilizingoptical photofabrication techniques. This typically involves thecontrolled projection of ultraviolet (UV) light through a mask (i.e.,reticle) and onto a layer of light-sensitive resist material depositedon a semiconductor wafer. The mask typically embodies a lighttransmissive substrate with a layer of light blocking material defininga pattern of circuit features to be transferred to the resist coatedwafer. If a negative acting resist is used, then the projected exposurelight passing through the mask will cause the exposed areas of theresist layer to undergo polymerization and cross-linking resulting in anincreased molecular weight. In a subsequent development step, unexposedportions of the resist layer will wash off with the developer, leaving apattern of resist material constituting a reverse or negative image ofthe mask pattern. Alternatively, if a positive acting resist is used,the exposure light passed through the mask will cause the exposedportions of the resist layer to become soluble to the developer, suchthat the exposed resist layer portions will wash away in the developmentstep, leaving a pattern of resist material corresponding directly to themask pattern. In both cases, the remaining resist will serve to define apattern of exposed semiconductor material that will undergo subsequentprocessing steps (e.g., etching and deposition) for forming the desiredsemiconductor devices.

The formation of circuit pattern features in the sub-micron rangerequires that a commensurate degree of resolution be obtained in theexposure step. Higher numerical light apertures and shorter lightwavelengths (e.g., deep UV range) yield higher resolution, but at theexpense of depth of focus. It is critical to increase as much aspossible the depth of focus of the projected image. Typically, exposurelight will be required to pass through relatively substantial resistmaterial thicknesses, and it is important that the mask pattern beaccurately projected throughout the depth of the resist material.Additionally, an increased depth of focus will minimize the adverseeffects of slight deviations of the exposure tool from a best focusposition (defocus conditions). Even the most precise photofabricationequipment cannot guarantee that sub-micron range deviations from a bestfocus position will not occur.

Recently, phase-shift masking techniques have been developed whichsignificantly increase resolution for a given depth of focus.Phase-shifting masks (PSM's) are distinguished from conventionalphotolithographic masks by the employment of selectively placed maskpattern materials allowing the transmission of exposure light with aphase-shift of π (180°). First pioneered in the early 1980's, suchtechniques holds great promise for extending the limits of conventionalphotolithography to the production of circuit features as small as 0.25μm, and perhaps smaller. The 180° phase difference created at the maskpattern edges sets up an interference effect that significantly enhancesedge contrast, resulting in higher resolution and greater depth of focus(as compared to the conventional binary intensity masks utilizing onlyan opaque mask pattern material, e.g., chrome). Advantageously, thetechnique can be employed utilizing conventional photolithographicstepper optics and resist techniques.

Numerous PSM techniques have been developed. These include alternating,subresolution, rim, and attenuated phase-shifting techniques. Seegenerally, C. Harper et al., Electronic Materials & Processes Handbook,2d ed., 1994, § 10.4, pp. 10.33-10.39. Of these, attenuatedphase-shifting techniques are among the most versatile, since they canbe applied to any arbitrary mask pattern. In attenuated PSM's, a singleslightly transmissive (halftone) absorber providing a phase-shift of180° can take the place of the conventional opaque, e.g., chrome, layerof mask pattern material. Originally, halftone materials were formed oftwo layers: a transmittance controlling layer and a phase controllinglayer. More recently, advantages have been realized through the use ofsingle layer materials developed to perform the dual function ofcontrolling light transmittance and phase-shift. As reported in Ito etal., Optimization of Optical Properties for Single-layer Halftone Masks,SPIE Vol. 2197, p.99, January 1994 (hereby incorporated by reference inits entirety), one such material comprises SiNx, wherein the compositionratio is controlled by changing the amount of flowing nitrogen.

Although attenuated PSM's have proven to be one of the most usefultechniques for applying actual device patterns with high resolution(see, e.g., K. Hashimoto et al., The Application of Deep UV PhaseShifted-Single layer Halftone reticles to 256 Mbit Dynamic Random AccessMemory Cell Patterns, Jpn. J. Appl. Phys. Vol. 33 (1994) pp. 6823-6830,hereby incorporated by reference in its entirety), new techniques arerequired to provide even greater resolution so as to allow feature sizesat and below 0.25 μm to be consistently produced with a low defect rate.Moreover, attenuated PSM's have not eliminated the problem of imageshortening effects.

Image shortening is a phenomena that reduces the attainable wholeresolution. With certain feature shapes, such as elongated holes used,e.g., to provide storage node, isolation and some contact hole levels inDRAM patterns, a slight defocus will result in a substantial shorteningof the hole images projected onto the underlying wafer. This resultsbecause, particularly in a defocus condition, e.g., ±1.0 μm, imageintensity and contrast tend to decrease considerably toward the ends ofthe holes. This is illustrated by the simulated image intensity contourplots of FIG. 1B, for a conventional attenuated PSM.

Accordingly, there is a need for semiconductor photofabrication maskstructures that will provide increased whole resolution and depth offocus, and which will minimize image shortening effects. There is also aneed for efficient methods of producing such masks.

SUMMARY OF THE INVENTION

In view of the foregoing, it is a principal object of the presentinvention to provide photofabrication mask structures that provideincreased whole resolution and depth of focus.

It is a further object of the present invention to providephotofabrication mask structures which will minimize image shorteningeffects.

Yet another object of the present invention is to provide efficientmethods of producing mask structures as aforesaid, particularly methodsthat can be performed with conventional semiconductor processingtechnologies.

These and other objects are achieved in accordance with a first aspectof the present invention by an attenuated phase shifting mask comprisinga light transmissive substrate and a light attenuating layer forming acircuit pattern on the substrate for projection onto a light sensitivematerial. The light attenuating layer comprises a primary region oflight transmittance t₁ and a secondary region of light transmittance t₂greater than t₁.

In another aspect, the invention is embodied in a method of making anattenuated phase shifting mask. Light attenuating material is arrangedin the form of a circuit pattern on a light transmissive substrate, toform a preliminary mask structure. A secondary region of the lightattenuating material is selectively partially oxidized such that thelight transmittance of the secondary region is increased relative to aprimary region of the light attenuating material.

In yet another aspect, the invention is embodied in an integratedcircuit photofabrication mask comprising a light transmissive substrate,a masking layer forming a circuit pattern on the substrate forprojection onto a light sensitive material and a surface topographyincluding a generally rounded light transmissive surface adjacent anedge of a circuit pattern feature. The generally rounded surface servesto diffract exposure light projected therethrough so as to create on thesemiconductor body an apparent mask boundary differing from an actualmask boundary.

In still another aspect, the invention is embodied in a method of makingan integrated circuit photofabrication mask. A masking layer is arrangedon a light transmissive substrate in the form of a circuit pattern to beprojected onto a light sensitive material to form a preliminary maskstructure. A generally rounded surface of light transmissive material isformed adjacent an edge of a circuit pattern feature, for diffractingexposure light projected therethrough so as to create on thesemiconductor body an apparent mask boundary differing from an actualmask boundary.

These and other objects, features and advantages of the presentinvention will be readily apparent and fully understood from thefollowing detailed description of the preferred embodiments, taken inconnection with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts simulated image intensity plots for exposure lightprojected through an elongated hole feature of a conventional attenuatedPSM, at best focus and defocus conditions.

FIG. 1B graphically depicts simulated exposure-defocus curves (an E-Dtree) for the elongated hole mask feature of FIG. 1A.

FIG. 2A depicts simulated image intensity plots for exposure lightprojected through an elongated hole feature of a mask made in accordancewith a first primary embodiment of the present invention, at best focusand defocus conditions.

FIG. 2B graphically depicts simulated exposure-defocus curves (an E-Dtree) for the elongated hole mask feature of FIG. 2A.

FIG. 3A depicts simulated image intensity plots for exposure lightprojected through an elongated hole feature of a mask made in accordancewith a second primary embodiment of the present invention, at best focusand defocus conditions.

FIG. 3B graphically depicts simulated exposure-defocus curves (an E-Dtree) for the elongated hole mask feature of FIG. 3A.

FIG. 4 is a diagrammatic top plan view illustrating an attenuated PSMhole pattern mask structure (positive resist case) in accordance withthe first primary embodiment of the present invention.

FIG. 5 is a diagrammatic top plan view illustrating an attenuated PSMline pattern mask structure (positive resist case) in accordance withthe first primary embodiment of the present invention.

FIG. 6A is a process flow chart illustrating a method for making themask structures of the first primary embodiment.

FIG. 6B presents top plan views illustrating certain of the processsteps shown in FIG. 4A, for the hole pattern mask structure of FIG. 4.

FIG. 7 is a partial cross-sectional view of a first mask structure inaccordance with the second primary embodiment of the present invention.

FIG. 8 is a diagrammatic close-up partial cross-sectional view of themask structure illustrated in FIG. 7, with exposure light projectedtherethrough.

FIG. 9 is a diagrammatic top plan view of the mask structure shown inFIG. 7, with exposure light passing therethrough.

FIG. 10 is a partial cross-sectional view of a second mask structure inaccordance with the second primary embodiment of the present invention.

FIG. 11 is a diagrammatic close-up partial cross-sectional view of thesecond mask structure shown in FIG. 10, with exposure light projectedtherethrough.

FIG. 12 is a diagrammatic top plan view of the second mask structureshown in FIG. 11, with exposure light projected therethrough.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 illustrates application of a first primary embodiment of thepresent invention to an attenuated PSM mask 1 including a patterncomprising elongated hole features 3. (It will be understood that theinvention is equally applicable to both single layer and multi-layermasks, as are known in the art). Mask 1 comprises a light transmissivesubstrate 5, e.g., of quartz. A light attenuating and phase shiftingmasking layer 7 is arranged on substrate 1 in the form of the desiredcircuit pattern.

Masking layer 7 is preferably a single layer of material that performsboth a light attenuating and phase shifting function, e.g., an SiNxcomposition as taught in the Ito et al. article mentioned in theBackground section. Alternatively, masking layer 7 may comprise separatelayers of light attenuating and phase shifting material, as are known inthe art. The transmittance of masking layer 7 is optimized for theparticular pattern, in accordance with known techniques.

The above-described structure of an attenuated PSM is modified in thepresent invention. In the illustrated embodiment, two regions 9a, 9b ofrelatively increased light transmissivity are created adjacent theopposite end portions of elongated hole feature 3. Whereas a remaining(primary) region of masking layer 3 may have a light transmissivity t₁,0<t₁ ≦20%, secondary regions 9a, 9b are provided with a lighttransmissivity t₂ (0<t₂ ≦20%) more than t₁. As one example, t₁ couldequal 8% and t₂ could equal 10% (a difference of 2%). The optimaldifference between t₁ and t₂ can be determined empirically, for a givenmask pattern, by simulated or actual exposure trials, as can the optimalpositioning and relative sizing of the secondary regions of increasedlight transmissivity. Preferably, the difference between t₂ and t₁ willnot exceed 10%, and the total area of the secondary regions will notexceed 1/3 the area of the primary regions. The shapes of the secondaryregions may vary, and may include rectangular, square, elliptical andcircular shapes.

As illustrated in the simulated image intensity plot of prior art FIG.1A, for an elongated hole feature of a conventional attenuated PSM, aslight defocus condition, e.g., ±1.0 μm, causes a considerable amount ofimage shortening (i.e., reduction in image contrast and intensity at theopposite ends of the elongated hole feature). The inventor hasrecognized that this problem is eliminated or substantially reduced byproviding regions of increased transmissivity adjacent the areas wherethe image shortening (a reduction of image intensity) occurs. For anelongated hole, this occurs at the opposite end portions of the hole.FIG. 2A illustrates that with a mask as shown in FIG. 4, little to noimage shortening occurs for the same defocus condition.

It is possible to reduce image shortening at the best focus positionwith oversizing of the mask features on conventional masks. Suchcompensation for mask bias does not solve the problem of imageshortening in a defocus condition. In contrast, image shortening atdefocus conditions can be reduced with the present invention. In thisrespect, the invention improves total lithographic performance comparedwith conventional masks.

Regions of relatively increased transmittance, as described above, alsoserve to increase the whole resolution (depth of focus for certain doselatitude) which is attainable. This is illustrated, for the elongatedhole feature, by a comparison of the simulated exposure-defocus curves(E-D tree) for a conventional attenuated PSM (see FIG. 1B) with that formask 1 (see FIG. 2B). In FIG. 1B, length curves 11a, 11b represent thelog of the doses required to fully expose the underlying resist materialalong the long sides (length dimension) of the elongated hole (to aspecified critical dimension (CD) criterion, e.g., 25 nm for 0.25 μmdesign rule, at different focus/defocus conditions. Point 12 on the Y(focus) axis represents a best focus position. Similarly, width curves13a, 13b represent the doses required to fully expose the underlyingresist material along the short sides (width dimension) of the elongatedhole.

The hatched region 15 defined by the overlap of the areas definedbetween the width curves and length curves represents the wholeresolution that is attainable. The maximum width of hatched region 15,measured along the y (focus) axis, represents the range of permissibledeviation from best focus point 12, i.e., the depth of focus. In FIG.2B, it can be seen that curves 11a', 11b' have far less curvature thancurves 11a, 11b of FIG. 1B, resulting in a substantially wider hatchedregion 15. Thus, it will be appreciated that the whole resolution anddepth of focus for mask 1 of the present invention is substantiallygreater than that for the conventional attenuated PSM.

FIG. 5 illustrates application of the above-described principles to anattenuated PSM mask 17 including a line feature 19 (positive resistcase). Similar to the embodiment of FIG. 4, the halftone masking layer21 is provided with a primary region of light transmittance t₁. Adjacentthe two end portions of the line feature are provided two secondaryregions 23a, 23b of increased light transmittance t₂. The ranges statedfor t₁ and t₂ for the elongated hole embodiment (FIG. 4) and therelative sizes of the primary and secondary regions thereof, are alsoapplicable to the line feature embodiment of FIG. 5. With thisarrangement, similar improvements in depth of focus, and correspondingdecreases in the image shortening effect are realized.

With reference to FIGS. 6A and 6B, a process for making a mask inaccordance with the present invention is now described. (As just onepossible example, FIG. 6B shows the processing of a mask having anelongated hole feature as shown in FIG. 4.) The first step 25 is theformation of a conventional halftone mask structure 27. This involvesthe arrangement of halftone masking layer 7 in the form of a circuitpattern on substrate 5, using conventional techniques such as coatingfollowed by electron beam or laser patterning.

Next, a resist material is applied to mask structure 27 and a patterningstep 29 (e.g., electron beam or laser beam patterning) is performed toremove the resist from the regions of the mask corresponding to thesecondary regions of increased transmissivity to be formed. This resultsin a resist pattern 31.

Next, in step 33, a weak oxidation treatment is carried out to partiallyoxidize the halftone material in the exposed secondary regions 34a, 34b.The oxidation of an SiNx halftone material will result in the formationof SiO₂ thus increasing light transmissivity. A weak oxidation treatmentis effective to increase the light transmissivity of other half tonephase shifting materials, e.g., MoSiOxNy, CrOx, C and Cr, and thehalftone layer (e.g., Cr) of multi-layer type attenuated phase shiftingmasks, e.g., Cr/SiO₂. The oxidation process should be carefullycontrolled in order to achieve the desired increase in lighttransmissivity. The oxidation agent should be chosen in light of theresist and halftone materials that are being used. The agent should beeffective to oxidize the halftone material at a controllable rate, whileat the same time leaving the resist layer intact. Two generally suitableoxidizing agents are O₂ assier (plasma) and sulfuric acid solution.

Finally, in step 35, the remaining resist material is removed, resultingin a finished mask structure 1 corresponding to that shown in FIG. 4.

A second primary embodiment of the invention is now described in termsof two preferred variations, one for an elongated hole feature, and onefor a line feature. As with the first primary embodiment describedabove, mask structures in accordance with the second primary embodimentserve to increase the attainable whole resolution and depth of focus,and to reduce image shortening effects. Additionally, the secondprincipal embodiment provides improved corner definition of theprojected feature and is applicable to both attenuated PSM andconventional opaque mask structures. It will be appreciated that thestructures of the second primary embodiment can be used alone, or incombination with the structures of the first primary embodiment.

In each of the variations, a generally rounded surface topography alongedges of the circuit features of the mask is used in order to diffractlight away from its normal path, in a manner that increases imageintensity and contrast. This compensates for a rounding that tends tooccur in photofabrication masks at the corners of circuit patternfeatures such as holes and lines. A certain degree of rounding isunavoidable with present mask fabrication techniques. Such roundingdegrades lithography performance by exasperating the image shorteningeffect.

FIGS. 7-9 illustrate the first variation, wherein the circuit patternfeatures include an elongated hole (positive resist case) 39. A mask 41includes a light transmissive substrate 43, e.g., of quartz, and apatterned layer of masking material 45. The masking layer may be eithera light blocking opaque material, e.g., chrome, or a light attenuatingand phase shifting material. If the latter, the layer may, as in thefirst principal embodiment, comprise either a single layer performing adual light attenuating and phase shifting function, i.e., SiNx, or twolayers performing these functions respectively.

As seen in FIG. 9, elongated hole 39 has a degree of rounding at itscorners 47. This rounding reduces corner definition and has a tendencyto cause image shortening on defocus (particularly in the lengthdirection of the feature).

In the present invention, an additional layer of light transmissivematerial 48 is deposited on top of substrate 43 and masking layer 45, inorder to create a generally rounded surface 46 overlying the edges ofmasking layer 45. Preferably, material 48 is an SiO₂ coating such as aspin-on-glass (SOG) film. Preferably, the thickness of layer 48 shouldgenerally not exceed the thickness of layer 45 by more than 50%. Such alayer has the effect of diffracting the exposure light 49 projectedtherethrough so as to create on the underlying resist coated wafer anapparent mask boundary 50 which is moved outwardly (typically a distanceδ equal to about 100 nm) with respect to the actual mask boundary 51.This, in turn, has the effect of increasing the light intensity in thecorners and improving corner definition.

As seen in FIG. 9, the size of the image projected by the mask is largerthan the actual size of the mask feature (about 100 nm on each side). Itis necessary to take this into account when determining the size of themask features and any amount of demagnification.

FIG. 3A shows simulated image intensity plots for the attenuated PSM ofFIGS. 7-9. It can be seen that for the best focus condition, greatlyimproved corner definition is obtained, as compared with the simulatedresults shown for a conventional attenuated PSM (FIG. 1A) and theattenuated PSM of FIG. 4 (FIG. 2A). In the defocus position, the resultis similar to that shown for the embodiment of FIG. 4, namely greatlyreduced image shortening relative to the conventional attenuated PSM (ora conventional binary mask).

FIG. 3B illustrates a simulated E-D tree for the attenuated PSM of FIGS.7-9. Similar to the result for the embodiment of FIG. 4, it can be seenthat curves 11a", 11b" have far less curvature than curves 11a, 11b ofFIG. 1B, resulting in a substantially wider hatched region 15" (measuredalong the y (focus) axis). Thus, it will be appreciated that the wholeresolution and depth of focus for mask 41 of the present invention issubstantially greater than that for the conventional attenuated PSM.

FIGS. 10-12 illustrate a second variation of the second primaryembodiment, wherein the circuit pattern includes a line feature 52(positive resist case). Similar to the first variation illustrated inFIGS. 7-9, a mask 53 includes a light transmissive substrate 55patterned with a layer of masking material 57. The mask materialalternatives (e.g., opaque vs. attenuated PSM) described with respect tothe first variation (FIGS. 7-9) apply also to the second variation.

As seen in FIG. 12, line 52 exhibits a degree of rounding at its corners59. As in the first variation, such rounding reduces corner definitionand has a tendency to cause image shortening, particularly in a defocuscondition.

In the second variation, mask substrate 55 is provided with a recessedregion 61 providing a generally rounded surface 62 underlying the edgesof the masking layer 57. Preferably, the recessed region is formed byetching the top surface of substrate 55 to a depth not exceeding threetimes (3x) the thickness of masking layer 57. To make an effectualrounded topography, an isotropic etching process (e.g., either wetetching or chemical dry etching (CDE)) can be used. Rounded surface 62has the effect of diffracting the exposure light 63 adjacent the lineedges inwardly. Light 63 is blocked (or attenuated and phase shifted) bymasking layer 57 so as to create an apparent mask boundary 65 which ismoved outwardly with respect to the actual mask boundary 67. This, inturn, improves corner definition and increases the light intensity andimage contrast in the corners of the line feature. Also, as in the firstvariation, a substantial increase in whole resolution and depth of focusis obtained.

The present invention has been described in terms of preferredembodiments thereof. Other embodiments, variations and modificationswithin the scope and spirit of the appended claims will occur to personsof ordinary skill in the art upon reviewing this disclosure.

I claim:
 1. An attenuated Phase shifting mask comprising:a lighttransmissive substrate; and a light attenuating layer forming a circuitpattern on said substrate for projection onto a light-sensitivematerial, the light attenuating layer comprising a primary region oflight transmittance t₁ and a secondary region of light transmittance t₂greater than t₁ ; wherein said circuit pattern comprises an elongatedhole feature and said secondary region of light transmittance is locatedadjacent opposite end portions of said elongated hole feature.
 2. Anattenuated phase shifting mask comprising:a light transmissivesubstrate; and a light attenuating layer forming a circuit pattern onsaid substrate for projection onto a light-sensitive material, the lightattenuating layer comprising a primary region of light transmittance t₁and a secondary region of light transmittance t₂ greater than t₁ ;wherein said circuit pattern comprises a line feature and a saidsecondary region of light transmittance is located adjacent opposite endportions of said line feature.
 3. An attenuated phase shifting maskcomprising:a light transmissive substrate; and a light attenuating layerforming a circuit pattern on said substrate for projection onto alight-sensitive material, the light attenuating layer comprising aprimary region of light transmittance t₁ and a secondary region of lighttransmittance t₂ greater than t₁ ; wherein 0<t₁, t₂ ≦20% and 0<t₂ -t₁≦10%.
 4. A mask according to claim 3, wherein t₁ is about 8% and t₂ isabout 10%.
 5. An attenuated phase shifting mask comprising:a lighttransmissive substrate; and a light attenuating layer forming a circuitpattern on said substrate for projection onto a light-sensitivematerial, the light attenuating layer comprising a primary region oflight transmittance t₁ and a secondary region of light transmittance t₂greater than t₁ ; wherein the secondary regions have a shape of one ormore of a rectangle, square, ellipse and circle, and the total of thesecondary region areas is equal to or less than 1/3 the area of theprimary region.